The present invention relates to a process for sterilization of medical instruments using a chemical sterilant and certifying that the instruments are sterile.
Medical instruments have traditionally been sterilized using either heat, such as is provided by steam, or a chemical, such as formaldehyde or ethylene oxide in the gas or vapor state. Each of these methods has its drawbacks. Many medical devices such as fiberoptic devices, endoscopes, power tools, etc., are sensitive to heat, moisture or both. Formaldehyde and ethylene oxide are both toxic gases that pose a potential hazard to healthcare workers. Problems with ethylene oxide are particularly severe, because its use requires long aeration times to remove the gas from articles that have been sterilized. This lengthens the sterilization cycle time undesirably.
Sterilization using liquid hydrogen peroxide solution has been found to require high concentrations of sterilant, extended exposure time and/or elevated temperatures. However, sterilization using hydrogen peroxide vapor has been shown to have some advantages over other chemical sterilization processes (see, e.g., U.S. Pat. Nos. 4,169,123 and 4,169,124, each of which issued Sep. 25, 1979, which are entitled respectively, xe2x80x9cHydrogen Peroxide Vapor Sterilization Methodxe2x80x9d and xe2x80x9cCold Gas Sterilization Processxe2x80x9d and which are incorporated herein by reference).
The combination of hydrogen peroxide with a plasma provides certain additional advantages, as disclosed in U.S. Pat. No. 4,643,876 issued Feb. 17, 1987 and entitled, xe2x80x9cHydrogen Peroxide Plasma Sterilization Systemxe2x80x9d which is incorporated herein by reference. Commercially available sterilization devices, such as the STERRAD(copyright) sterilization systems sold by Advanced Sterilization Systems division of Ethicon, Inc. automate the process of injecting a solution of hydrogen peroxide into a sterilization chamber, vaporizing the solution to provide a hydrogen peroxide vapor, contacting articles to be sterilized with the vapor, and exciting the vapor into the plasma stage. The hydrogen peroxide for each sterilization cycle is shipped to the location of the sterilization system, generally by air or ground transportation.
Preferably, as in the case with the STERRAD(copyright) brand systems, pre-measured amounts of a hydrogen peroxide and water solution are provided in sealed enclosure, such as a capsule inside of a cassette housing which can be automatically opened by the system to reduce contact between the system user and the hydrogen peroxide solution. Such cassettes are described more fully in U.S. Pat. No. 4,817,800 issued Apr. 4, 1989 entitled, xe2x80x9cFluid Injection System Cassette and Fluid Packaging Methodsxe2x80x9d and U.S. Pat. No. 4,899,519 issued Feb. 13, 1990 with the same title, each of which are incorporated herein by reference.
The sterilization of articles containing diffusion-restricted areas, such as long narrow lumens, presents a special challenge. Methods that use hydrogen peroxide vapor that has been generated from an aqueous solution of hydrogen peroxide have certain disadvantages. One disadvantage is that because water has a higher vapor pressure than hydrogen peroxide, it will vaporize faster. Another disadvantage is that because of its lower molecular weight, water will diffuse faster than hydrogen peroxide in the vapor state. Because of these physical properties, when an aqueous solution of hydrogen peroxide is vaporized in the area surrounding the items to be sterilized, the water reaches the items first and in higher concentration. The water vapor more quickly diffuses into and thus inhibits penetration of hydrogen peroxide vapor into diffusion-restricted areas, such as small crevices and lone narrow lumens. Simply employing a more concentrated solution of hydrogen peroxide fails to adequately address the problem due to the difficulty in handling highly concentrated hydrogen peroxide solutions. Transportation of such solutions can be particularly difficult. In general, such solutions are limited to concentrations of less than 60% hydrogen peroxide, however, regulations and the like regarding such concentrations may of course be modified in the future. In any event, shipping and handling of highly concentrated solutions remains impractical.
U.S. Pat. No. 4,952,370 issued Aug. 28, 1990 and entitled xe2x80x9cHydrogen Peroxide Sterilization Methodxe2x80x9d and incorporated herein by reference discloses a sterilization process in which aqueous hydrogen peroxide vapor is first condensed on the article to be sterilized, followed by application of a vacuum to the sterilization chamber to remove the water and hydrogen peroxide from the article. This method is suitable for surface sterilization, but not for sterilization of diffusion-restricted areas such as long narrow lumens because it depends on the diffusion of hydrogen peroxide vapor into the lumen to effect sterilization.
U.S. Pat. No. 4,943,414 issued Jul. 24, 1990 and entitled xe2x80x9cMethod for Vapor Sterilization of Articles Having Lumensxe2x80x9d discloses a process in which a vessel containing a small amount of a vaporizable liquid sterilant solution is attached to a lumen, and the sterilant vaporizes and flows directly into the lumen of the article as the pressure is reduced during the sterilization cycle. This system has the advantage that the water and hydrogen peroxide vapor are pulled through the lumen by the existing pressure differential, increasing the sterilization rate for lumens, but has the disadvantage that the vessel needs to be attached to each lumen to be sterilized.
U.S. Pat. No. 5,492,672 issued Feb. 20, 1996 and entitled, xe2x80x9cSterilization Apparatus and Method for Multicomponent Sterilantxe2x80x9d discloses a process for sterilizing narrow lumens. This process uses a multi-component sterilant vapor and requires successive alternating periods of flow of sterilant vapor and discontinuance of such flow. A complex apparatus is used to accomplish the method. Because flow through of vapor is used, closed end lumens are not readily sterilized in the process.
U.S. Pat. No. 4,744,951 issued May 17, 1988 to Cummings and entitled xe2x80x9cVaporization met,hod to Enhance Sterilant Penetrationxe2x80x9d attempts to address this problem by providing a separate prechamber connected to the sterilization chamber. Hydrogen peroxide is first admitted to the prechamber where it is concentrated in a distillation procedure employing the differing vapor pressures of hydrogen peroxide and water. Water""s higher vapor pressure allows one to select a vaporization pressure that selectively vaporizes water from a hydrogen peroxide solution, thus concentrating the solution. Cummings pumps air out of the prechamber and lowers its pressure to a level at which the water preferentially vaporizes from the hydrogen peroxide solution. The pump that is evacuating the prechamber draws out the water vapor thus released from solution to concentrate the remaining solution. To prevent the water vapor from traveling into the narrow spaces such as endoscope lumens, Cummings carries out the concentration process in the prechamber which is physically isolated from the main chamber. This adds complexity by requiring additional chambers, pumps and valves.
Those of skill in the art would not think to employ such a concentration process in the same chamber as the sterilization occurs. Such a process first draws the water out of solution and it would have been thought that this water vapor would simply enter and thus occlude the narrow lumens, thereby inhibiting the later diffusion of hydrogen peroxide, no matter how concentrated, into those lumens. However, the present inventors have surprisingly found that concentrating the hydrogen peroxide vapor within the sterilization chamber greatly increases the ability to sterilize long narrow lumens over the conventional process.
An additional advantage of the ability to concentrate the hydrogen peroxide is the ability to accurately predict the outcome of a sterilization cycle. Normally, some form of biological indicator containing a test microorganism is included with a load of instruments to be sterilized and the load is not certified as being sterile and ready for use without first checking to see whether the microorganisms in the biological indicator are killed. Applicants have surprisingly found that by sufficiently concentrating the hydrogen peroxide and monitoring that the concentration is achieved that the sterilization process is so predictable as to be able to release a load as sterilized without the need for a further biological indicator reading, i.e. parametric release.
The present invention comprises a method of sterilizing, and certifying as sterile, an article. It includes the steps of:
a) placing the article into a sterilizer;
b) introducing hydrogen peroxide and water into the sterilizer
c) vaporizing the hydrogen peroxide and water to form a vapor comprising hydrogen peroxide and water;
d) determining the concentration of hydrogen peroxide in the vapor;
e) determining the concentration of water in the vapor;
f) selectively drawing water vapor from the sterilizer to increase the ratio of hydrogen peroxide to water in the sterilizer;
g) repeating steps c)-f) until the ratio of hydrogen peroxide to water is at a desired level; and
h) furnishing the vaporized hydrogen peroxide to the article for a sufficient time to effect sterilization thereof and then certifying the sterility of the article based upon achieving the desired level.
Preferably, such desired level is chosen from the group consisting of:
i) attaining a ratio of hydrogen peroxide to water of at least 0.1 to 1 by weight,
ii) attaining a ratio of hydrogen peroxide to water in the vapor which is at least two times higher than the ratio of hydrogen peroxide to water which is introduced into the sterilizer in step b),
iii) attaining a concentration of hydrogen peroxide and water of at least 60% by weight of hydrogen peroxide, and
iv) attaining a hydrogen peroxide concentration of at least 0.45 mg/L.
The sterilizer may comprise a diffusion-restricted area. The sterilizer may comprises a chamber and an enclosure, with the enclosure in fluid communication with the chamber. The hydrogen peroxide and water are introduced into the sterilizer via the enclosure. The enclosure may also comprise a diffusion restricted area.
In one aspect of the invention the ratio of hydrogen peroxide to water introduced into the sterilizer is less than 0.1 to 1 by weight. Preferably, the sterilizer is evacuated to a pressure below the atmospheric pressure, more preferably to a pressure below the vapor pressure of the hydrogen peroxide and water in solution.
Preferably, the concentration of hydrogen peroxide occurs in solution so that after the ratio of hydrogen peroxide to water is at the desired level a portion of the hydrogen peroxide remains in liquid form and is then vaporized.
In one aspect of the invention, a plasma is generated in the sterilizer.
Preferably, the temperature of unvaporized hydrogen peroxide and water in the sterilizer is monitored to more accurately control the vaporizing process.
In one aspect of the invention, the load contains a maximum challenge equivalent to sterilizing the center of a lumen 1 mm in diameter and 400 mm long, and in another equivalent to sterilizing the center of a lumen 1 mm in diameter and 250 mm long.
In one aspect of the invention the hydrogen peroxide vapor is furnished to the article for a period of at least 15 minutes, or alternatively for at least 30 minutes.